vefd: Thin wrapper around eventfd with pipe emulation

[dridi]

We recently made change to the HTTP/2 implementation in Varnish Enterprise and would like to share this change. The new HTTP/2 architecture relies on a non-portable eventfd() notification system over a file descriptor, to enable polling of both the client socket and h2_req threads notifications simultaneously from the h2_sess thread.

We settled on this minimal API to capture our limited eventfd() usage and added a fallback to pipe() decided at configure time for systems without eventfd() support.

[nigoroll]

bugwash consensus: wait for h2 patch which uses this to make a judgement if it can be avoided